In the extraction of oil from earth boreholes, the naturally existing pressure within an earth formation is often used as the driving force for oil extraction. The oil may be extracted from a single location or "zone" within the well, or oil may be extracted from multiple zones within the well. In either case, it is desirable to know the fluid pressure within the well at multiple locations to aid the well operator in maximizing the depletion of the oil within the earth formation.
It is often required to provide some form of artificial pumping power to force the oil being extracted up the borehole to the surface where it can be collected. In such producing wells, electrically powered pumps located at the bottom of the wells are typically employed. Such devices, called electrical submersible pumps (ESPs), are typically installed after the well has been drilled and while it is being put into production. ESPs are located at the bottom end of a long length of tubing, called the production tubing string, and are powered by electrical cables deployed from the surface.
It is known to install electrical pressure and temperature sensors with some ESPs to provide the operator on the surface with information about the pump's performance. The collected information then allows the operator to control various parameters, such as pump speed, which can increase the life of the pump. Increasing the pump life until a scheduled maintenance, when other scheduled downhole work can be accomplished at the same time, is highly desirable since it minimizes costs due to lost oil production.
The presently used electrical pressure and temperature sensors are limited for several reasons. The on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration and high levels of external hydrostatic pressure. Such electrical sensors also must be extremely reliable, since early failure may entail a very time consuming and expensive well intervention. Electronics, with its inherent complexity, are prone to many different modes of failure. Such failures have traditionally caused a less than acceptable level of reliability when these electrical sensors are used to monitor ESPs.
There are numerous other problems associated with the transmission of electrical signals within wellbores. In general, there are problems encountered in providing an insulated electrical conductor for transmitting electrical signals within wellbores. Such electrical conductors must be sealed against exposure to wellbore fluids, which are at high temperatures, high pressures, and present a very corrosive environment. Such electrical conductors, once damaged by the fluids which penetrate the insulating materials around the electrical conductors, will typically short electrical signals. Additionally, electrical transmissions are subject to electrical noises in some production operations.
It is also known to use optical interferometers for the measurement of wellbore conditions, such as downhole wellbore pressures and temperatures. However, optical interferometers are typically very sensitive to temperature variations and the downhole temperature of a specific position within a wellbore will change over time, depending upon different factors such as, for example, production rates, the types of fluids produced over the life of the well, and downhole wellbore conditions. Even optical interferometers designed of special material or construction are subject to inaccuracies because of the harsh borehole environment and because of the very tight tolerances in such precision equipment. Additionally, such optical interferometers are located at the end of an optical fiber and are only useful for making a single measurement at the point within the system the sensor is located.
Therefore, a reliable system is needed for accurately measuring the pressure of a harsh environment, such as a borehole. Additionally, such a system should be capable of measuring pressure at multiple locations within the harsh environment